A fuel cell comprises, in general, a stack of individual cells within which an electrochemical reaction takes place between two, continuously introduced reactants. The fuel, such as hydrogen for fuel cells operating with hydrogen/oxygen mixtures, or methanol for fuel cells operating with methanol/oxygen mixtures, is brought into contact with the anode, while the oxidant, generally oxygen, is brought into contact with the cathode. The anode and cathode are separated by an electrolyte in the form of an ion-conducting membrane. The electrochemical reaction, the energy from which is converted into electrical energy, is broken down into two half-reactions:                oxidation of the fuel, taking place at the anode/electrolyte interface and producing, in the case of hydrogen fuel cells, protons H+, which will pass through the electrolyte in the direction of the cathode, and electrons, which rejoin the external circuit, so as to contribute to the production of electrical energy;        reduction of the oxidant, which takes place at the electrolyte/cathode interface, with production of water, in the case of hydrogen fuel cells.        
The electrochemical reaction takes place, properly speaking, within a membrane electrode assembly.
The membrane electrode assembly is a very thin assembly, with a thickness on the millimetre scale, and each electrode is supplied with the gases, for example, by means of a corrugated plate.
The ion-conducting membrane is generally an organic membrane containing ionic groups which, in the presence of water, allow conduction of the protons produced at the anode by oxidation of the hydrogen.
The thickness of this membrane is generally between 50 and 150 μm and is the result of a trade-off between mechanical strength and ohmic loss. This membrane also allows separation of the gases. The chemical and electrochemical resistance of these membranes allows them in general to operate in the fuel cell for durations of more than 1000 hours.
The polymer making up the membrane must therefore fulfil a certain number of conditions in relation to its mechanical, physicochemical and electrical properties, including the conditions defined below.
The polymer must first of all be able to give thin films, of 50 to 150 micrometres, which are dense and defect-free. The mechanical properties, modulus of elasticity, breaking stress and ductility, must make the polymer compatible with the assembly operations, including, for example, an operation of clamping between metal frames.
The properties must be retained on passing from the dry state to the wet state.
The polymer must have high thermal stability to hydrolysis and must exhibit high resistance to reduction and to oxidation. This thermomechanical stability is assessed in terms of variation in ionic resistance, and in terms of variation in mechanical properties.
The polymer must, lastly, possess a high ion conductivity, this conductivity being provided by acid groups, such as carboxylic acid, phosphoric acid or sulphonic acid groups, which are bonded to the chain of the polymer.
For a number of decades, different types of proton-conducting polymers have been proposed that can be used for constituting fuel-cell membranes.
Employed first of all were sulphonated phenolic resins prepared by sulphonating polycondensed products, such as phenol-formaldehyde polymers.
The membranes prepared with these products are inexpensive, but lack sufficient stability to hydrogen at 50-60° C. for long-term applications.
Attention then turned to sulphonated polystyrene derivatives, whose stability is greater than that of the sulphonated phenolic resins, but which cannot be used at more than 50-60° C.
At present, acceptable performance properties are obtained on the basis of polymers composed of a perfluorinated linear main chain and a side chain that bears a sulphonic acid group.
Among these best-known polymers, which are available commercially, mention may be made of the polymers registered under the Nafion® brand names from the company Du Pont de Nemours.
However, the membranes used at present, and especially the Nafion® membranes, have a service temperature limit of the order of 90° C., exhibit a phenomenon of ageing after 3000-4000 hours of service, and, lastly, possess inadequate proton conduction.
The problem addressed by the inventors was therefore that of providing membranes which eliminate the aforementioned drawbacks, and, more particularly, membranes which exhibit better proton conduction than the existing membranes such as the Nafion® membranes.